Foil air bearings are known for use with high-speed air rotating shafts. A machine with foil air bearings is more reliable than one with rolling element bearings because it requires fewer parts to support the rotating assembly and needs no lubrication. In operation, an air/gas film between a bearing and a rotating shaft protects the foil bearing itself from wear. The bearing surface is in contact with the shaft only when the machine starts and stops, and a coating on the foils limits wear at those times.
The principle of an air bearing, whether of the journal or thrust type, is simple. When two surfaces form a wedge and one surface moves relative to the other, pressure is generated between the surfaces due to the hydrodynamic action of the fluid carrying the load. In a journal bearing, the shaft deflects and a wedge is formed due to the eccentricity between the shaft center and the bearing center.
Even though the principle of an air bearing is simple, application is complex. For instance, in a normal journal bearing assembly, the running radial clearance between the shaft and bearing is extremely small (typically less than 0.0005 inch for a 2-inch-diameter shaft at 36,000 rpm, for example). Any eccentricity in the shaft, or friction within the bearings, may cause shaft deflection and/or shaft thermal expansion that could exceed the running clearance, thereby reducing the useful life of the bearing assembly. In addition, damping is required to suppress any whirl instability, and there can be misalignment between various rotating and stationary parts within the assembly.
Foil bearings address these problems. While the shaft is stationary, there is a small amount of preload between the shaft and the foil bearing. As the shaft turns, hydrodynamic pressure is generated between the shaft and the bearing foils, pushing the foils away from the shaft and making the shaft completely airborne. This phenomenon occurs nearly instantly during start-up, and at a very low speed. When the shaft is airborne, the friction loss due to shaft rotation is extremely small. As the shaft expands or deflects, the foils get pushed farther away, keeping an air film clearance relatively constant. In addition, the foils provide coulomb damping due to frictional contact therebetween, which enhances the rotational stability.
An exemplary air foil bearing assembly 20 is shown partially exploded in FIG. 1. The bearing assembly 20 contains a thin layer of top foil 22 supported on a corrugated or “bump” foil 24. The bump foil 24 is arranged on the inner circumferential surface 26 of the bearing housing 28, and the top foils 22 is inserted on the inner annular surface 30 of the bump foil 24. At least an inner annular surface 32 of the top foil 22 is typically coated with a solid film lubricant to provide low contact friction between a rotating shaft (not shown) and the inner annular surface 32 of the top foil 22.
The bump foil inner annular surface 30 is in frictional contact with an outer annular surface 34 of the top foil 22, providing support to the top foil 22. The corrugations 36 of the bump foil 24, as well as the thickness of the bump foil 24, are designed to provide a desired stiffness, and spring force between the bearing housing inner circumferential surface 26 and the top foil 22 to provide the desired bearing load support capacity. Typically, a small amount of preload is desired between the shaft (not shown) and the inner annular surface 32 of the top foil 22 when the shaft is at rest. During shaft rotation, air is drawn between the shaft and the top foil inner annular surface 32, where it is compressed. Due to hydrodynamic action, the compressed air deflects the top foil 22 away from the shaft and the shaft is supported by a cushion of air. As the top foil 22 deflects radially outwardly, it is supported by the corrugations 36 of the bump foil 24. Depending upon the magnitude of the hydrodynamic forces, the corrugations 36 elastically deform, thereby providing a compliant feature of the bearing assembly 20. In particular, the pre-determined spring rate of the bump foil 24 accommodates shaft expansion, shaft excursion and housing misalignment. The corrugations 36 also provide a flow path for a small amount of cooling air, thereby maintaining a desired temperature in the bearing assembly 20.
Typically, both the top foil 22 and the bump foil 24 are separately stamped from sheet metal having a desired thickness. The bumps 36 may be formed in the bump foil 24 as part of the stamping process, or they may be formed in a second stamping or rolling operation. The ends 40 of the top foil 22 are formed with a ninety degree flange that is affixed to a metallic key 42, usually by a spot welding or other bonding operation. Similarly, the ends 44 of the bump foil 24 are each formed with a ninety degree flange, and are affixed to the metallic key 42 by a spot welding or other bonding operation, wherein the bump foil is located between the top foil 22 and the inner circumferential surface 26 of the bearing housing 28 when assembled into the housing 28 (FIG. 2). The key 42, including the ends 40, 44 respectively of the top foil 22 and the bump foil 24, is then inserted into a machined keyway 46 formed in the bearing housing 28. Insertion of the key 42 into the keyway 46 prevents rotational movement of the top foil 22 and the bump foil 24, which is crucial to proper air bearing operation. To prevent axial migration of the foils 22, 24 within the bearing housing, plates 48 are then attached to the outer surface 29 of the bearing housing 28, typically by spot welding or other bonding operation, wherein the plates 48 cover the ends of the keyway 46. Machining the keyway, welding each of the ends 40, 44 respectively of the top foil 22 and the bump foil 24 to the key 42, and welding the plates 48 to the outer surface 29 of the bearing housing 28 are complex, time-consuming manufacturing operations. It would be desirable to develop an air foil bearing requiring less complex manufacture while retaining the desirable bearing and functional characteristics.